The Facts

The flu is a respiratory (i.e., nose, throat, and lung) infection that can be caused by a variety of influenza viruses. Many people use the word "flu" when they actually have a cold. Although the common cold is also caused by viruses, the flu and common cold differ in several ways.

In North America, the flu almost always strikes between November and April. Up to 25% of the population may be infected in an average year. Stronger epidemics (i.e., when the flu occurs in more people than expected in a given area or season) come every 2 or 3 years, infecting twice as many people as during an "off" year.

Most people who get the flu will recover within 1 to 2 weeks, but some people are at risk of developing complications such as pneumonia. On average, about 4,000 to 8,000 people in Canada die each year from complications of influenza, and about 20,000 people with the flu are hospitalized. Most of these people have other medical conditions, are seniors, or are very young children.

Causes

Influenza is contagious, which means it can be spread easily from person to person. Viruses that cause influenza spread from person to person mainly by droplets of respiratory fluids sent through the air when someone infected with the virus coughs or sneezes. Other people inhale the airborne virus and can become infected.

In some cases, the flu can be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) that has the virus on it and then touches his or her nose, mouth, or eyes. The flu is most easily spread in crowded places such as schools and offices.

There are three families of influenza virus: A, B, and C. Type C more commonly affects ducks, geese, turkeys, and chickens, but it has also been involved in a small percentage of human cases. Type B mainly affects humans and causes a milder disease, and it changes very little from year to year.

Type A influenza poses the most serious problems for humans. Strains of this type have also been found in birds, humans, horses, pigs, seals, whales, and ferrets. Viruses that affect two different species sometimes combine and mix-and-match genetic information to create a new strain that nobody is immune to and for which no vaccine has been prepared.

There are potentially infinite new varieties of type A influenza. Avian flu is a type of influenza A that had been seen only in birds, including chickens and ducks, until 1997 when the first human case was reported in Hong Kong. Although the avian flu affected mostly animals, it did cause a few cases of severe disease with a high risk of death in humans. Many birds were slaughtered in Hong Kong to prevent the disease from spreading. In early 2004, outbreaks of the avian flu re-emerged in Japan, South Korea, Vietnam, Thailand, Cambodia, China, and Indonesia. Human cases have been reported in Vietnam and Thailand. Millions of chickens have been affected and killed in the countries listed in an attempt to stop the spread of the disease.

Swine flu is another type of influenza A that is normally found in pigs. Swine flu is passed from pig to pig, and although it usually only infects pigs, there have been periodic infections in humans, resulting in H1N1 flu virus (human swine flu). Most of these cases occur in people with direct exposure to pigs (e.g., people working on pig farms). Person-to-person transmission of the H1N1 flu virus (human swine flu) does occur, but it is not clear how easily the virus is spread among people.

The flu takes 1 to 4 days to incubate in humans, but infected people become contagious before symptoms appear, often just the day after the virus enters the body. Adults remain infectious (i.e., they can spread the virus to others) for about 6 days, and children remain infectious for up to 10 days.

Symptoms and Complications

Initial flu symptoms include headaches, chills, and a cough. Symptoms such as fever, loss of appetite, and muscle aches soon follow. Other symptoms such as nausea, vomiting, and diarrhea are rare in adults but more common in children.

Since many people think they have the flu when it's actually a bad cold, here's a quick guide to help you tell the difference:

Symptom

Cold

Flu

fever

uncommon

usually present, high (38°C to 41°C or 102°F to 104°F); lasts 3 to 4 days

headache

uncommon

very common

aches and pains

slight

common and often severe

fatigue and weakness

mild

significant; can last 2 to 3 weeks

extreme exhaustion

never

very common at the start

stuffy nose

common

sometimes

sneezing

common

sometimes

sore throat

common

sometimes

chest discomfort and cough

mild to moderate, hacking cough

common; often severe with painful cough

For most people, the flu lasts 1 or 2 weeks, but it can last for up to 1 month. The main complications are secondary bacterial infections of the sinuses or lungs (pneumonia). Symptoms include fever; chills; and yellow, green, or brown sputum (nasal discharge). Children are prone to ear infections like otitis media.

People in nursing homes are at a higher risk of complications from flu because they may have weak immune systems and often have other medical problems. People with asthma, chronic obstructive pulmonary disease, or congestive heart failure are also at a higher risk of developing bacterial infections like pneumonia.

In American studies, influenza hospitalization rates for children under 5 years of age were second only to the hospitalization rate in people over 65 years of age. School-aged children have the highest infection rates both during and between epidemics, and they are likely to be infected early in the season. Households with school-aged children have infection rates 30% above the average rate.

Making the Diagnosis

Since the symptoms of an influenza infection are very different from the common cold, the diagnosis can be made fairly quickly. Your doctor will be able to tell that you have the flu if you have at least some or most of these symptoms:

aches and pains everywhere, especially in the back and legs

bad headache

burning sensation in the chest

dry cough at first, then bringing up sputum

high fever

nausea and possible vomiting

sore throat

runny or stuffy nose

extreme tiredness

If there is any doubt, your doctor can make sure that it's the flu by taking a throat swab and testing it for the virus. This is rarely needed.

Treatment and Prevention

The normal treatment for flu is rest and plenty of liquids. Treatment also includes ways to prevent spreading the flu virus, such as proper handwashing, keeping common surfaces clean, and coughing or sneezing into your arm or sleeve.

Medications for specific symptoms can help. Cough suppressants can be used for cough. Acetylsalicylic acid* (ASA), ibuprofen, or acetaminophen can be used to treat symptoms of the flu, such as aches and fever. Children and teenagers with flu shouldn't take ASA or other salicylates (medications related to ASA, such as salsalate or magnesium salicylate). The combination of influenza and ASA is linked to Reye's syndrome, a rare but serious condition affecting the brain and liver. Many over-the-counter cold medications contain ASA or other salicylates. Ask your doctor or pharmacist about this.

Antibiotics are not effective against viral infections like flu and the cold, but they are prescribed for complications such as bacterial infections.

Antiviral medications are sometimes used to treat the flu. These medications can help shorten the duration of the flu and reduce symptoms if they are taken within 2 days of the start of symptoms. Antiviral medications are also recommended to prevent flu infection for some people. Antivirals can be used to prevent flu in children and adults after they come into close contact with a person who has the flu, such as flu-infected people who live in the same household. Generally, this is not recommended for most people; however, antivirals may be recommended for people at risk for flu complications. In these situations, antiviral medications should be started as soon as possible after becoming exposed to the person with the flu. Your doctor can decide whether you should start antiviral medications.

Amantadine is an antiviral but is usually not recommended for influenza A treatment because many of these viruses are resistant to it. However, if tests show the virus to be sensitive to amantadine, it can shorten the duration of symptoms if taken within 24 to 48 hours of symptoms appearing. It can also be used to prevent type A flu in certain circumstances. It carries some risk of side effects, including insomnia and confusion. Amantadine does not work against type B viruses.

Zanamivir and oseltamivir are antiviral medications that can be used to treat and prevent influenza A. They prevent newly formed viruses from escaping the infected cells that produced them. This limits further spread of the virus in the body. Zanamivir is an inhaled spray, whereas oseltamivir is a pill. Taken within 24 to 48 hours after the onset of illness, these medications reduce the duration of symptoms by an average of 1 to 3 days.

Flu antibodies can prevent the flu. The only ways to generate antibodies are to be infected or to get vaccinated. Because the flu viruses can change from year to year, vaccination needs to be repeated every year.

Each spring, a worldwide network of physicians and testing labs decide which flu strains are likely to cause trouble and design that year's vaccine accordingly. The vaccine gives resistance to the type B strain and the two type A strains that are expected to predominate in the coming flu season.

The effectiveness of the vaccine can vary from season to season, which means there is still a chance that you can get the flu – although your symptoms may be milder. The flu vaccine is given to anyone classified as high risk for flu complications, and for people who are caregivers, health workers, and anyone who wants to avoid the flu. The flu vaccine is paid for by the government in some Canadian provinces, and universal vaccination is encouraged.

High-risk groups for flu complications include:

anyone aged 65 years or older

young children under 5 years old (especially if they are less than 2 years old)

organ transplants: steroids, medications that suppress the immune system to prevent it from rejecting the organ (e.g., cyclosporine, tacrolimus, mycophenolate mofetil)

cancer: chemotherapy

certain types of arthritis, such as rheumatoid arthritis: steroids, biologics (medications that modify the response of the immune system, such as adalimumab or infliximab), medications that suppress the immune system to prevent it from attacking the body (e.g., methotrexate, azathioprine)

Crohn's disease: steroids, biologics (see above), or medications to suppress the immune system

blood disorders (e.g., anemia, sickle cell anemia)

neurologic and neurodevelopmental disorders that affect their ability to swallow and breathe

morbid obesity (BMI of 40 or higher)

residents of nursing homes or other chronic care facilities, regardless of age

children receiving long-term ASA therapy

pregnant women (especially if they are in their second or third trimester) and women who were recently pregnant (within the last 6 weeks)

Aboriginal peoples

People who should not receive a flu shot include children less than 6 months of age and people who have had an allergic reaction to a previous flu vaccine.

The injectable flu shot (but not the nasal spray) has been shown to be safe for many people with egg allergies. Your doctor will need to assess whether you should have a flu shot if you are allergic to eggs. Be sure to tell your health care provider about this and any other allergies you may have before you are given your flu shot.

You can reduce your risk of getting the flu by washing your hands regularly using soap and warm water or an alcohol-based hand sanitizer. Also, cough or sneeze into a tissue or into your sleeve. Dispose of the tissue right away. If you have flu symptoms, stay home from work or school and avoid contact with people who are at a high risk of flu complications (e.g., seniors, nursing home residents).

*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.